1. Field of the Invention
The present invention is generally related to a magnetic sensor and, more particularly, to a magnetic sensor that is able to set a comparative threshold limit which is determined as a function of a peak value in order to optimize the operation of the sensor over a wide range of gaps between the sensing face of the sensor and the path of a ferromagnetic object.
2.Description of the Prior Art
Many different types of geartooth sensors are well known to those skilled in the art. U.S. Pat. No. 5,442,283, which issued to Vig et al on Aug. 15, 1995, discloses a Hall voltage slope-activated sensor circuit that includes a Hall element that is preferably followed by a Hall voltage amplifier and a pole end of a magnet which is preferably fixed adjacent to the Hall element. The amplifier output is connected directly to one of a pair of differential inputs of a Schmitt trigger circuit and is also connected, via a single or a dual polarity track and hold circuit, to the other of the differential Schmitt inputs. The dual polarity track and hold circuit causes the voltage across a capacitor to track positive and negative Hall voltage slopes and to hold the positive going peaks and negative going peaks of the Hall voltage presented to the other Schmitt input so that when the difference voltage between the Hall voltage and the held voltage of the capacitor exceeds a positive or negative threshold of the Schmitt circuit, the Schmitt circuit output changes binary state indicating the approaching edge or the receding edge of a ferrous geartooth. The dual polarity track and hold circuit is made up of two complimentary plus and minus peak detector circuits, each including a bipolar transistor having an emitter connected to the voltage holding capacitor, an operational amplifier with an output connected to the base of the transistor, the amplifier having a negative input connected to the transistor emitter and a positive input serving as the input of the Hall voltage tracking and holding circuit.
U.S. Pat. No. 5,500,589, which issued to Sumcad on Mar. 19, 1996, describes a method for calibrating a sensor by moving a magnet while monitoring an output signal from a magnetically sensitive component. The sensor is provided with a carrier that has a cavity shaped to receive a magnet in sliding association therein. Ribs are provided to guide the movement of the magnet into the cavity and a deformable rib is used to hold the magnet at a precise position determined by an active calibration process. A magnetically sensitive component is rigidly attached to a substrate and the substrate is rigidly attached to the carrier in which the cavity is formed. Electrically conductive leads are molded into the carrier and extend through the carrier to positions where they can be electrically connected to circuit runs on the substrate. A flexible wall can also be deformed in the carrier to deflect in response to the insertion of a magnet into the cavity. This provides additional holding capability that retains the magnet in position when an external force is removed.
U.S. Pat. No. 5,497,084, which issued to Bicking on Mar. 5, 1996, discloses a geartooth sensor with a means for selecting a threshold magnitude as a function of the average and minimum values of a signal of magnet field strength. The geartooth sensor is provided with a circuit which determines a threshold magnitude as a function of the minimum value of a first output signal from a magnetically sensitive component and an average output signal from a magnetically sensitive component. Circuitry is provided to determine the average signal. The minimum signal is then subtracted from the average signal and the resulting signal is doubled before being scaled by a predetermined fraction and then compared to the original output signal from the magnetically sensitive component. This circuit therefore determines a threshold signal as a function of both the minimum signal value and the average signal value and, in addition, enables the resulting signal to be scaled to a predetermined percentage of this difference for the purpose of selecting a threshold value that is most particularly suitable for a given application.
U.S. Pat. No. 5,414,355, which issued to Davidson et al on May 9, 1995, describes a magnet carrier disposed within a outer housing. A magnetic sensor is provided with a housing in which a carrier is inserted. The carrier is particularly shaped to retain a permanent magnet in a particular position relative to a plurality of electrical conductors and a substrate on which a magnetically sensitive component is attached. The carrier and its associated components are inserted into a housing which can be deformed to permanently retain the carrier within a cavity of the housing. All of the components of the sensor are designed to be easily assembled along a common axis to facilitate automatic assembly and manufacture of the sensor. The magnetically sensitive component can be a Hall effect element that is associated with other electrical components which are also attached to a substrate that is disposed proximate to a front end of the carrier.
The skilled artisan is familiar with the general operation of most magnetic sensors which typically comprises steps of measuring an output, from a magnetically sensitive component, such as a Hall effect element or a magnetoresistive device, and comparing that voltage to a predetermined threshold value. The threshold value can, in certain applications, be a constant magnitude. However, changing conditions can significantly affect the signal from the magnetically sensitive component and, generally, the accuracy of the sensor can be improved by calculating the threshold magnitude as a function of the changing signals from the magnetically sensitive component. For example, U.S. Pat. No. 5,497,084 measures the minimum magnitude of the signal from a Hall element and the average magnitude of the signal from the Hall element and uses these two variable signals to determine an appropriate threshold level. The Hall signal is then continuously compared to the threshold signal to determine the presence or absence of ferromagnetic teeth passing through a detection zone of the sensor. U.S. Pat. No. 5,442,283, in a different approach to this general problem, uses the peak values of the Hall signal as a temporary reference value and then changes the binary output signal from the sensor when the Hall signal decreases by a preselected amount relative to the stored peak value. U.S. Pat. No. 5,414,355 and U.S. Pat. No. 5,500,589 are typical of the type of structure used in magnetic sensors. These four patents mentioned above are expressly incorporated by reference in this description.